It is well known that ultrasonic imaging comprises a potentially valuable diagnostic tool, for example in studies of the vascular system, particularly in cardiography, and of tissue microvasculature. A variety of contrast agents has been proposed to enhance the acoustic images so obtained, including suspensions of solid particles, emulsified liquid droplets, gas bubbles and encapsulated gases or liquids. It is generally accepted that low density contrast agents which are easily compressible are particularly efficient in terms of the acoustic backscatter they generate, and considerable interest has therefore been shown in the preparation of gas-containing and gas-generating systems, e.g. to contrast agents which incorporate or are adapted to incorporate volatile substances.
Much attention has been given to the selection of gases for gas-containing ultrasound contrast media in order to enhance properties such as their stability and duration of echogenic effect. Thus, for example, WO-A-9305819 proposes use of free microbubbles of gases having a coefficient Q greater than 5 where EQU Q=4.0.times.10.sup.-7.times..rho./C.sub.s D
(.rho. being the density of the gas in kg.m.sup.-3, C.sub.s the water solubility of the gas in moles.l.sup.-1 and D the diffusivity of the gas in solution in cm.sup.3.sec.sup.-1). An extensive list of gases said to fulfill this requirement is presented, this including a variety of hydrocarbons such as alkanes, alkenes etc. and halogenated compounds such as sulphur hexafluoride, disulphur decafluoride, tungsten hexafluoride, brominated and/or chlorinated and/or fluorinated and/or iodinated hydrocarbons, brominated aldehydes, fluorinated ketones, fluorinated amines, fluorinated mercaptans etc.
EP-A-0554213 suggests that one may impart resistance against collapse under pressure to gas-filled microvesicles by introduction thereto of at least one gas whose solubility in water, expressed in litres of gas/litre of water under standard conditions, divided by the square root of its molecular weight does not exceed 0.003. Gases said to be preferred include halogenated compounds such as sulphur hexafluoride, selenium hexafluoride and various Freons.RTM..
WO-A-9416739 discloses ultrasound contrast agents comprising liquid-in-liquid colloidal dispersions in which the dispersed phase is a liquid having a boiling point below the body temperature of the subject to be imaged. Such agents thus undergo a phase change to an echogenic gaseous foam following administration to the subject. Examples of suitable volatile dispersed phase liquids are said to include aliphatic hydrocarbons such as butanes and pentanes, and organic halides, in particular fluorocarbons such as perfluorobutane, perfluoropentane and perfluoroneopentane.
WO-A-9503835 proposes use of microbubbles containing a gas mixture the composition of which is based on considerations of gas partial pressures both inside and outside the microbubbles, so as to take account of osmotic effects on microbubble size. Representative mixtures comprise a gas having a low vapour pressure and limited solubility in blood or serum (e.g. a fluorocarbon) in combination with another gas which is more rapidly exchanged with gases present in normal blood or serum (e.g. nitrogen, oxygen, carbon dioxide or mixtures thereof).
WO-A-9516467 suggests use of ultrasound contrast media containing a mixture of gases A and B, where gas B is present in an amount of 0.5-41% v/v, has a molecular weight greater than 80 daltons and has aqueous solubility below 0.0283 ml/ml water under standard conditions. Representative gases B include halogenated gases such as sulphur hexafluoride and a variety of brominated and/or chlorinated and/or fluorinated hydrocarbons. Representative gases A, which comprise the balance of the mixture, include air, oxygen, nitrogen, carbon dioxide and mixtures thereof.
WO-A-9608234 claims containers comprising an aqueous lipid suspension phase and a gaseous phase, e.g. comprising a hydrocarbon such as an alkane or alkene or, more preferably, a fluorinated gas such as a perfluorocarbon, which is substantially separate from the lipid phase. Agitation of the contents of such containers is said to produce gas-filled liposome compositions which are useful as contrast agents in, for example, ultrasonic or magnetic resonance imaging.
WO-A-9618420 is directed to products comprising gas bubbles immobilised within a frozen physiologically acceptable aqueous carrier medium containing appropriate stabilising additives etc.; such products may be thawed to generate injectable ultrasound contrast agents. Representative gases are said to include halogen-containing gases such as sulphur hexafluoride, perfluoromethane, perfluoroethane, perfluoroethene, perfluoropropane, perfluoropropene, perfluorobutane, perfluorobut-2-ene, perfluorobutadiene, perfluorocyclobutane, perfluoropentane, perfluorocyclopentane and mixtures thereof.
Contrast agents such as those disclosed in the above-mentioned EP-A-0554213, WO-A-9503835 and WO-A-9516467 are typically intended to be supplied as dry formulations, e.g. comprising appropriate surfactants, additives, stabilisers etc., under an atmosphere of the intended gas content. Such dry formulations may be reconstituted by addition of an appropriate carrier liquid, e.g. sterile pyrogen-free water or saline, prior to administration, e.g. by intravenous injection.
Contrast agents such as those of WO-A-9416739 and WO-A-9608234 will be supplied as liquid formulations. Those of WO-A-9608234 specifically require the presence of gas in the headspace above the aqueous lipid phase. Any headspace above stored forms of contrast agents according to WO-A-9416739 may contain some dispersed phase material in vapour form by virtue of the stated volatility of this material.
In general contrast agents and precursors therefor such as dry formulations will typically be supplied in closed containers, for example in vials or other vessels closed with closure means such as stoppers or septa, or in pre-filled syringes. The contents of such containers may advantageously be such as to constitute or generate an appropriate unit dose of the contrast agent. Containers such as vials may typically comprise glass and/or plastics materials and be closed with closure means comprising plastics materials, for example elastomers. Containers such as syringes may likewise comprise glass and/or plastics material and typically may incorporate plungers comprising plastics material.